In the integrated circuit (IC) industry, manufacturers are currently imbedding dynamic random access memory (DRAM) arrays on the same substrate as CPU cores or other logic devices. This technology is being referred to as embedded DRAM (eDRAM). Embedded DRAM generally can provide microcontroller (MCU) and other embedded controllers faster access to larger capacities of on-chip memory at a lower cost than that currently available using conventional embedded static random access memory (SRAM) and/or electrically erasable programmable read only memory (EEPROM).
A semiconductor memory, such as a DRAM or embedded DRAM, mainly consists of a transistor and a capacitor. Therefore, improvement in the efficiency of these two structures tends to be the direction in which technology is developing. DRAM is generally a volatile memory, and the way to store digital signals is decided by charge or discharge of the capacitor in the DRAM. When the power applied on the DRAM is turned off, the data stored in the memory cell completely disappears. A typical DRAM cell usually includes at least one field effect transistor (FET) and one capacitor. The capacitor is used to store the signals in the cell of DRAM. If more charges can be stored in the capacitor, the capacitor has less interference when the amplifier senses the data. In recent years, the memory cell of a DRAM has been miniaturized more and more from generation to generation. Even if the memory cell is minimized, a specific charge is essentially stored in the storage capacitor of the cell to store the information.
When the semiconductor enters the deep sub-micron process, the size of the device becomes smaller. For the conventional DRAM structure, this means that the space used by the capacitor becomes smaller. Since computer software is gradually becoming huge, even more memory capacity is required. In the case where it is necessary to have a smaller size with an increased capacity, the conventional method of fabricating the DRAM capacitor needs to change in order to fulfill the requirements of the trend.
There are two approaches at present for reducing the size of the capacitor while increasing its memory capacity. One way is to select a high-dielectric material, and the other is to increase the surface area of the capacitor. There are two main types of capacitor that increase capacitor area. These are the deep trench-type and the stacked-type, where digging out a trench and filling the trench with a conductive layer, a capacitive dielectric layer and a conductive layer in sequence for the capacitor form the deep trench-type capacitor.
When a dielectric material with a relatively high dielectric constant is used in a stacked capacitor, the materials for manufacturing the upper and the bottom electrodes need to be gradually replaced in order to enhance the performance of the capacitor. A structure known as a metal-insulator-metal (MIM) structure possesses a low-interfacial reaction specificity to enhance the performance of the capacitor. Therefore, it has become an important topic of research for the semiconductor capacitor in the future.
Cell areas are reduced, as a semiconductor device needs ultra-high integrity. Thus, many studies for increasing the capacitance of a capacitor are being developed. There are various ways of increasing the capacitance such as forming a stacked or trench typed three-dimensional structure, whereby a surface area of a dielectric layer is increased.
In order to constitute a cell area in a DRAM fabrication, transistors and the like are formed on a semiconductor substrate, storage and plate electrodes of polycrystalline silicon and a dielectric layer are formed wherein the dielectric layer lies between the electrodes, and metal wires are formed to connect the devices one another.
The obtainable capacitance of the storage capacitor tends to decrease dependent upon the level of the miniaturization of the storage cell. On the other hand, the necessary capacitance of the capacitor is almost constant when the storing voltage to be applied across the capacitor is fixed. Therefore, it is necessary for the capacitor to compensate the capacitance decrease due to the miniaturization by, for example, increasing the surface area of the capacitor. This surface area increase has been popularly realized by increasing the thickness of the lower electrode (or, storage electrode) of the capacitor. A typical capacitor utilized in DRAM fabrication is the Metal Insulator Metal (MIM) capacitor, which is usually located in the memory region of DRAM and embedded DRAM to increase the capacitance of the capacitor.
A capacitor is thus generally one of the most useful of passive components that is commonly integrated with active bipolar or CMOS transistors in modern VLSI devices. Integrated capacitors are commonly fabricated between polysilicon (i.e. PIP capacitors) poly to polycide/metal (i.e. MIS capacitors) or metal-to-metal (i.e. MIM) capacitors. All of these types of capacitors may be planar in nature for process compatibility and simplicity.
The MIM capacitor provides superior advantages for mixed-signal/RF applications than other PIP or MIS capacitors. An MIM capacitor is typically fabricated initially in the BEOL (back-end manufacturing) and only requires low process temperatures (i.e., less than 450 C), so that a minimum disturbance of transistor parameters is present. Additionally, MIM capacitors offer excellent linearity and symmetry due to the lack of the so-called “depletion effect,” which is generally evidenced with PIP or MIS capacitors. MIM capacitors thus are fully compatible with logic processes and are preferred for modern mixed-signal or RF applications.
In present MIM formation processes and fabrication operations for embedded DRAM devices, the total number of additional lithographic steps in the BEOL (i.e., back end manufacturing process) is about 2-3, which is generally inefficient, particularly for foundry applications. As such, present MIM formation processes do not permit improvements in capacitance of MIM capacitor without additional BEOL steps. Based on the foregoing, the present inventors have concluded that a need exists to improve the capacitance of MIM capacitors and that such an improvement can be obtained by incorporating copper fabrication processes into the formation of MIM capacitors for embedded DRAM devices.